NASA spacecraft trying to get into Mercury's orbit

March 17, 2011

This image released by NASA shows an enhanced photo image of Mercury from its Messenger probe’s 2008 flyby of the planet. NASA says it was a taste of pictures likely to come after March 17, 2011, when the probe enters Mercury’s orbit. This photo shows the eastern part of the smallest and closest planet in our solar system. The colors in this picture are different than what would be seen with the naked eye, but show information about the different rock types and subtle color variations on the oddball planet. The bright yellow part is the Caloris impact basin, which is the site of one of the biggest in the solar system. Earth is about to get better acquainted with its oddball planetary cousin. (AP Photo/NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington)

A desk-sized NASA spacecraft is riding the brakes all the way to Mercury, about to pull a tricky maneuver Thursday night to become the first man-made object to orbit the tiny planet.

After a trip of 4.9 billion miles and nearly six-and-a-half years, the Messenger spacecraft will try to careen into an egg-shaped orbit and fight off the gigantic gravitational pull of the sun. To do so, it will have to use more than half of the fuel it was launched with in 2004 to reduce speed dramatically in just fifteen minutes.

If all works well, Messenger will circle as close as 120 miles from the surface and survey the entire planet for a year, beginning April 4. It will learn about Mercury's mysterious magnetic field and - most tantalizing of all - discover if the closest-planet-to-the-sun has ice in its permanently dark frigid craters near its poles.

If all doesn't go well, it is more likely that the $446 million spacecraft flies away from Mercury and ends up circling the sun. NASA should know whether they succeed in about an hour.

Making it difficult has been the sun's gravity and its incredible heat, forcing the probe to have large sunshades, said mission systems engineer Eric Finnegan. Messenger has had to take the long way around to get into Mercury's orbit because it has to continuously slow down and burn energy to get to the right place. While other spaceships have whipped around planets like a slingshot to speed up, Messenger has had to do just the opposite, going around Earth, Venus and Mercury to slow down.

"You're playing this incredible game of cosmic billiard balls to make this all work out," said Johns Hopkins University astronomer Ralph McNutt, who is a mission scientist.

If something goes wrong, engineers have fallback plans they can use in the next three days to three months to try to get the spacecraft back in some kind of orbit around Mercury.

NASA is shuttering the cameras on the spacecraft during the maneuver so there will be no pictures of the arrival, said mission chief scientist Sean Solomon.

Messenger has flown by Mercury a few times in its torturous trip there and Mariner 10 flew by there 36 years ago.

"Mercury has been called the forgotten planet because it's been so long since we sent a spacecraft there," Solomon said.

MESSENGER is the first mission sent to orbit the planet closest to the sun. On Oct. 6, 2008, at roughly 4:40 a.m. ET, MESSENGER flew by Mercury for the second time this year. During the encounter, the probe swung just 125 ...

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Yes, the ice thing is surprising for me too. Really shows how light and heat are linked...proximity to the heat source doesn't seem to be as big a factor... Yes, I agree--satellites to study all of the planets (maybe instead of the numerous war-based ones we have).

Actually, it is likely to be at least partially water. IF I understand it correctly, I believe one theory is that gasses like hydrogen, oxygen and carbon could be released from rocks on the surface due to solar heating. Some of those gasses would drift into the dark craters where they would instantly condense and snow down to the crater floor. Over a long enough time it could be a significan amount of ice. If the temperature inside the craters is around -150 C then that's plenty cold enough for both water and CO2 ice to build up. With no significant atmosphere, once ice forms in a crater there would be nothing to ever remove it.

"Actually, it is likely to be at least partially water. IF I understand it correctly, I believe one theory is that gasses like hydrogen, oxygen and carbon could be released from rocks on the surface due to solar heating."

Yeah, the 2 significant sources for these ices would be outgassing and impacts from comets and asteroids, similar to what is proposed for Earth's Moon*.

I'm not really up on just what ices are probable (and non-ices are possible), but comets are known to be good sources of H2O (and the depolarized nature of the radar reflections is noted as an indicator of water ice).

"With no significant atmosphere, once ice forms in a crater there would be nothing to ever remove it."

My NSSDC* source notes:

"There are mechanisms for potential loss of ice, however. These include photodissociation, solar wind sputtering, and micrometeoroid gardening. The effects of these processes are not well-understood."